The field of the present invention relates to optical systems for driving motors such as dithering or oscillating motor drive for producing a scanned beam in a data reading application.
Data reading devices, such as bar code scanners, read symbols such as those found on consumer and industrial products, including one-dimensional codes such as UPC code, EAN/JAN, Code 39 or two-dimensional codes such as PDF-417. Scanners may be stationary, handheld or combination stationary/handheld scanners. Typically a data reading device such as a bar code scanner illuminates a bar code and senses light reflected from the code to detect the bars and spaces of the code symbols and thereby derive the encoded data. In a common system, an optical beam of light, such as a laser beam produced by a laser diode is scanned over a scan angle so as to scan the laser spot across the item being read.
In applications requiring rapid scanning of an illumination beam, methods employed for rapidly and repetitively scanning the illumination beam across a scanned region include mirror dithering such as described in U.S. application Ser. No. 08/934,487 and light source dithering such as described in U.S. Pat. No. 5,629,510, both of which are hereby incorporated by reference. Dithering, i.e. rapid rotational oscillation of an illumination beam, causes the illumination beam to move rapidly back and forth generating a scan line. When this scan line illuminates a barcode, the resulting time dependent signal due to detected light scattered and/or reflected from the bars and spaces of the barcode is decoded to extract the information encoded therein.
FIG. 1 illustrates a dithering assembly 100 comprising an oscillating structure which has a resonant frequency determined by the effective spring constant of bending member 112 and the effective mass of the mirror/magnet assembly 110 and any components attached thereto. The dithering assembly 100 comprises a mirror/magnet assembly 110, drive coil 106, feedback coil 108, bending member 112, and mounting member 114. The mirror/magnet assembly 110 comprises mirror 102, mirror bracket 103, drive magnet 104 and feedback magnet 105. The drive coil 106, feedback coil 108 and mounting member 114 may be part of or mounted within a housing (not shown) for dithering assembly 100. The bracket 103 holds mirror 102 and is connected to mounting member 114 by bending member 112, which may comprise a thin, flat sheet of flexible material which acts as a bendable spring.
Bending of member 112 results in pivoting/rotation of mirror/magnet assembly 110 about an axis substantially parallel to mirror 102, perpendicular to the plane of FIG. 1. The motion of mirror/magnet assembly 110 is driven by passing an oscillating drive current through drive coil 106 thereby generating an oscillating magnetic driving force on drive magnet 104. The maximum amplitude of dithering motion of the mirror 102 occurs when the drive current oscillates at the resonant frequency of dithering assembly 100, i.e., when the dithering assembly 100 is driven resonantly. It is important to drive the dithering assembly 100 resonantly to obtain the maximum dithering amplitude with minimum drive power consumption. It is also important that the position and length of the resulting scan line remain constant.
A feedback coil 108 positioned adjacent the mirror bracket 103 experiences an oscillating magnetic field due to motion of the feedback magnet 105, which is attached to bracket 103. The electrical potential developed across feedback coil 108 varies directly with time derivative of the magnetic flux at feedback coil 108, and hence with the velocity of feedback magnet 105 and dithering mirror 102. The zero crossings of the feedback potential, which occur when the mirror velocity is zero, are used to trigger switching of the polarity of the drive current in drive coil 106, thereby reversing the drive force exerted on drive magnet 104 and mirror 102. In this manner, the switching frequency of the drive force matches the resonant the frequency of the dithering motion of dithering assembly 100 and the drive force is in phase with the velocity for a resonantly driven system.
It is also possible to derive velocity feedback from a fixed Hall Effect sensor mounted adjacent a moving magnet, or from a piezoelectric element attached to the flexure such as disclosed in U.S. application Ser. No. 08/934,487.
There are several disadvantages with these feedback schemes such as requiring additional sensing hardware and control electronics, which add to the overall power consumption, cost, and/or complexity of the scanning system.
The present invention is directed to systems and methods for driving motors, such as those motors used to dither scan mechanisms in a scan module or scanning assemblies therefore. A preferred embodiment is directed to a resonantly driven dithering assembly employing feedback such as for scanning an illumination beam for a barcode scanner in which the velocity feedback signal is derived from the back-EMF of the actuator motor coil. The drive current is pulsed for a given duration commencing at a start point where the velocity of the dither mechanism is measured to be zero.